Adds Oaktree's quote in paragraph 4, details and background throughout

Nov 7 (Reuters) - Investment fund Oaktree Capital Management has urged British drugmaker Indivior INDV.L to cooperate with the fund in refreshing its board of directors, it said in a letter dated Nov. 6 and published in a filing on Thursday.

The request from the fund, which became the company's second largest shareholder in July, comes weeks after Indivior issued its second profit and revenue warnings in a span of three months, citing lower than expected sales from its opioid addiction treatment.

The company's opioid addiction treatment Sublocade has been losing market share due to the rapid adoption of Brixadi, a competing drug developed by Swedish pharmaceutical firm Camurus CAMX.ST.

"The Company essentially disregarded Brixadi’s entrance into the market by failing to take basic steps to protect Sublocade’s competitive position," Oaktree said in the letter.

Shares in the British drugmaker have slumped about 40% so far this year.

Oaktree, which specialises in providing rescue funding to struggling firms, had a 7.37% stake in the drugmaker, according to LSEG data.

Investment manager Two Seas Capital is the top shareholder with a 10.25% ownership.

Indivior did not immediately respond to a Reuters request for comment.

In July, the drugmaker had taken a $86 million hit in a settlement over its alleged role in spreading opioid addiction across the United States.

(Reporting by Yamini Kalia and Aby Jose Koilparambil in Bengaluru; Editing by Mrigank Dhaniwala)

((Yamini.Kalia@thomsonreuters.com;))